4^ Syjiem of the World. — Pendulums.-— Planetary Atmofpheres. 



ries it through fpace with: the planetary fyftem, unlefs this mottoti be fup pofed to have 

 been deftroyed by an impulfe in the oppofite direction ; a circumftance by no means 

 probable.- 



' " '/he impulfe given to an homogeneous fphere, in a dire£l:ion which does not pafs 

 through its centre, will caufe it to revolve conflantly round the diameter, which is perpendi- 

 cular to a plane pafTmg tlirough its centre, and the line of direflion of the imprefled force. 

 New forces afling on all its parts, and of which the refult paffcs through its centre, will 

 not change the parallelifm of its axis of rotation. Thus it is that the axis of the earth 

 remains always nearly parallel to itfelf-in its revolution round the fun, without its being 

 neceflary to fuppofe, with Copernicus, an annual motion of the poles of the earth round 

 ihofe of the ecliptic. 



*' If the body poflefs a certain figure, its axis of rotation may change every inftant. The 

 determination of thefe changes, whatever may be the forces afting on the bodies, is one of 

 the molt interefling problems of mechanics refpefting hard bodies, on account of its con- 

 nexion with the preceflion of the equinoxes, and the libration of the moon. The folution 

 of this problem has led to a curious and very ufeful refult ; namely, that in all bodies there 

 cxift three axes perpendicular to each other, round which the body may turn uniformly 

 when not folicited by external forces. On this account thefe axes have been called principal 

 axes of rotation. 



" A body or fyflem of bodies, poflefling weight, and of any figure whatever, ofcillating 

 round a fixed and horizontal axis, forms a compound pendulum. No other pendulum 

 cxifts in nature. The fimple pendulums fo frequently treated of are pure geometrical con- 

 ceptions, proper to Amplify the objects of difcuffion. It is eafy to refer to thefe fuch com- 

 pound pendulums as have their parts immoveably fixed together. If the length of the 

 fimple pendulum, whofe ofcillations are ifochronou^ with thofe of the compound pendulum, 

 be multiplied by its total mafs, and by the diftance of it« centre of gravity from the axis of 

 ofcillation, the produft will be equal to the fum of the produiSts of each particle of the 

 compound pendulum, multiplied by the fquare of its diftance from the axis. It is by 

 means of this rule, difcovered by Huyghens, that experiments with compound pendulums 

 have been applied to (hew the length of the fimple pendulum, which beats feconds." 



The author enters into a confiderable detail refpe£ling the atmofpheres of the planets. 

 " In all the changes to which the atmofpherc is fubjed: (fays he, vol. ii. p. 128.) the fum 

 of the produdls of the particles of the revolving body and its atmofphere, multiplied re- 

 fpedlively by the areas they defcribe round the common centre of gravity, the radii being 

 projedted on the plane of the equator, remain the fame in equal times. Suppofing, there- 

 fore, that, by any caufe whatever, the atmofphere fliould become contradled, or that part 

 thereof fliould become condenfed on the furface of the body, the rotatory motion of the 

 body and its atmofphere would be accelerated : for, the radii veSores of the areas de- 

 fcribed by the particles of the original atmofphere becoming fmaller, the fum of the pro- 

 du6ls of all the particles, by their correfponding areas, cannot remain the fame unlefs the 

 vetecity be augmented. 



" The atmofphere is flattened towards the poles, and fwelled out at the equator. But 

 this oblatenefs has its limits ; and in the cafe where it is greateft, the ratio of the polar and 

 equatorial diameter is as two to three. 



- . . " The 



